Communications systems are used to transfer data between remote transmitting and receiving locations over one or more physical channels. The integrity and reliability of such data transmissions can be compromised by several factors, such as transmission imperfections, receiving imperfections, and physical channel interference. Examples of interference which affect the quality and rate of data transmission include impulses (or bursts), and other short duration events, and long duration interference, such as ingresses. A relatively large effort has been directed to improving the transmitter and receiver robustness for long duration interference. However, compensating for impulse/burst noise has received little attention. Impulse or burst noise can be caused by many uncontrollable events, such as arcing and electrical transients coupled through a power system, lightning, etc. This type of noise typically occurs at unexpected times and lasts for a relatively short period of time (on the order of several microseconds). Because it is broad band in effect, impulse noise is of particular concern in multi-carrier transmission schemes since the impulse noise may affect many, or even all channels simultaneously.
For example, in a multi-tone modulation transmission scheme, a number of carriers positioned at different frequencies are used and data is transmitted simultaneously in parallel over the carriers. Each band carries a fraction of the total information being transmitted. The discrete bands or sub-channels are independently modulated, and each has a carrier frequency at the center frequency of the particular band. For example, multi-tone modulation schemes include DMT (Discrete Multitone modulation), used for DSL channel communication, and VCMT (Variable Constellation Multi Tone), suggested for use in HFC CATV applications.
In one DMT configuration, a 1.1 MHz channel is broken down into 256 sub-channels or bands, each of which is 4 KHz wide. Each sub-channel has its own carrier frequency. Each of the sub-channels is used to transmit a fixed number of information bits in a single symbol or signal period. The DMT system monitors the signal to noise ratio for each of the sub-channels and uses this information to determine how many bits per signal period (symbol) may be carried in each of the sub-channels. The number of bits per signal in a sub-channel is typically referred to as the loading of the sub-channel. The DMT system dynamically adjusts the loading of each of the sub-channels in accordance with the noise characteristics of the sub-channel. If a sub-channel is particularly noisy, it may not be used at all.
VCMT modulation is a transmission scheme specifically designed to effectively combat the high ingress and impulse/burst impairments present in cable TV channels, and also to maximize the data throughput capacity of such channels. Like DMT, data is transmitted over multiple sub-channels, typically 36. However, in VCMT, data is transmitted using a variable bit loading per tone, along with coding and interleaving. The VCMT system measures the noise present on each channel and independently modulates the tone data transmission scheme from QPSK (quadrature phase shift keying) to 256-QAM (quadrature amplitude modulation) accordingly. During operation, the signal-to-noise ratio across the channels are monitored for each tone and the headend receiver instructs the upstream transmitter in the cable modem to modify the QAM constellation for each tone to maintain a desired bit error rate. VCMT also uses spectral shaping to reduce the frequency sidelobes of the tones to thereby reduce the effect of narrowband interference to only the closest tones.
Multi-tone transmission schemes have relatively good noise immunity. However, there is a tradeoff when resistance to impulse noise is considered. Because data is transmitted on multiple channels, the symbol length can be relatively long while still providing a high overall data transmission rate. Both DMT and VCMT have relatively long symbol periods. Thus, each has fairly good immunity (compared to a single carrier signal) from weak time domain events, such as impulses and bursts, because the effect of a short time domain event will be averaged out over the relatively longer symbol period. However, strong impulses are very damaging to DMT transmission because the impulse may simultaneously corrupt all of the 256 symbols occupying the 256 DMT sub-channels. Similarly, strong impulses are also very damaging to VCMT transmissions because the impulse can corrupt the symbols occupying every VCMT sub-channel at the time the impulse hits. In addition, because VCMT uses spectral shaping, neighboring symbols may also be corrupted by the impulse.
Impulses can be corrected by either canceling or blanking. Impulse canceling is a process where the impulse waveform is estimated and then subtracted from the data signal at the appropriate time. Impulse blanking is a process in which the impulse is located in time and the data input is zeroed (or blanked) for the duration of the impulse. Canceling is superior to banking because it preserves the underlying data. However, it is considerably more complex to implement.
Various techniques for compensating for impulse noise are known. U.S. Pat. No. 5,410,264 to Lechleider, which is incorporated herein by reference, describes very basic techniques for impulse detection, timing, and cancellation. The techniques are based on match filtering with a known impulse shape. An impulse is detected by using a match filter and monitoring when the filter output energy crosses a defined threshold. The location of the impulse is estimated to be the single match filter output sample which has the maximum energy. However, the techniques disclosed in the '264 patent are incapable of handling impulses having more than one degree of freedom (e.g., spanned by a basis of more than one vector).
U.S. Pat. No. 5,479,440 to Esfahani, which is incorporated herein by reference, discloses an impulse cancellation technique which compensates for out-of-band impulse noise by filtering out frequency components suspected as impulses. However, the '440 patent does not adequately address in-band impulse noise. U.S. Pat. No. 5,703,904 to Langberg, which is incorporated herein by reference, discusses temporarily inhibiting (e.g., blanking) modem adaptation circuits when an impulse is detected. However, impulses are detected only by using a simple thresholding input signal. Further, no attempt is made to cancel the impulse. A somewhat more detailed impulse detection scheme, using an adaptive threshold, is disclosed in U.S. Pat. No. 5,119,321, which is incorporated herein by reference.
However, none of these prior art impulse detection and compensation schemes provides a robust and efficient impulse detection system which can adequately detect impulses which have two or more unknown (or varying) degrees of freedom. Further, none discloses an efficient scheme for handling strong impulses in a multi-channel transmission system.
Accordingly, it is an object of the present invention to provide a method for compensating for strong impulse interference in a multi-carrier system, such as DMT and VCMT.
It is a further object of the invention to provide an improved technique for detecting and timing impulses which have attributes that span a number of degrees of freedom.
Yet another object of the invention is to provide a system which will reliably detect an impulse, determine its attributes, and then take appropriate corrective action.
Yet a further object of the invention is to provide an impulse detection and correction system which is suitable for in band impulse noise as well as out-of-band impulses.
A further object of the invention is to detect and correct non-stationary noise, i.e., noise which has statistics which change over time.